Example source code (ImageSliceIteratorWithIndex.cxx):

// \index{Iterators!and image slices}
// The \doxygen{itk}{ImageSliceIteratorWithIndex} class is an extension of
// \doxygen{itk}{ImageLinearIteratorWithIndex} from iteration along lines to
// iteration along both lines \emph{and planes} in an image.
// A \emph{slice} is a 2D
// plane spanned by two vectors pointing along orthogonal coordinate axes.  The
// slice orientation of the slice iterator is defined by specifying its two
// spanning axes.
// \begin{itemize}
// \index{itk::Image\-Slice\-Iterator\-With\-Index!SetFirstDirection()}
// \item \textbf{\code{SetFirstDirection()}}
// Specifies the first coordinate axis
// direction of the slice plane.
// \index{itk::Image\-Slice\-Iterator\-With\-Index!SetSecondDirection()}
// \item \textbf{\code{SetSecondDirection()}}
// Specifies the second coordinate axis
// direction of the slice plane.
// \end{itemize}
// Several new methods control movement from slice to slice.
// \begin{itemize}
// \index{itk::Image\-Slice\-Iterator\-With\-Index!NextSlice()}
// \item \textbf{\code{NextSlice()}} Moves the iterator to the beginning pixel
// location of the next slice in the image.  The origin of the next slice is
// calculated by incrementing the current origin index along the fastest
// increasing dimension of the image subspace which excludes the first and
// second dimensions of the iterator.
// \index{itk::Image\-Slice\-Iterator\-With\-Index!PreviousSlice()}
// \item \textbf{\code{PreviousSlice()}} Moves the iterator to the \emph{last
// valid pixel location} in the previous slice.  The origin of the previous
// slice is calculated by decrementing the current origin index along the
// fastest increasing dimension of the image subspace which excludes the first
// and second dimensions of the iterator.
// \index{itk::Image\-Slice\-Iterator\-With\-Index!IsAtReverseEndOfSlice()}
// \item \textbf{\code{IsAtReverseEndOfSlice()}} Returns true if the iterator
// points to \emph{one position before} the beginning pixel of the current
// slice.
// \index{itk::Image\-Slice\-Iterator\-With\-Index!IsAtEndOfSlice()}
// \item \textbf{\code{IsAtEndOfSlice()}} Returns true if the iterator points
// to \emph{one position past} the last valid pixel of the current slice.
// \end{itemize}
// The slice iterator moves line by line using \code{NextLine()} and
// \code{PreviousLine()}.  The line direction is parallel to the \emph{second}
// coordinate axis direction of the slice plane (see also
// Section~\ref{sec:itkImageLinearIteratorWithIndex}).
// \index{itk::Image\-Slice\-Iterator\-With\-Index!example of using|(}
// The next code example calculates the maximum intensity projection along one
// of the coordinate axes of an image volume.  The algorithm is straightforward
// using ImageSliceIteratorWithIndex because we can coordinate
// movement through a slice of the 3D input image with movement through the 2D
// planar output.
// Here is how the algorithm works.  For each 2D slice of the input, iterate
// through all the pixels line by line. Copy a pixel value to the corresponding
// position in the 2D output image if it is larger than the value already
// contained there.  When all slices have been processed, the output image is
// the desired maximum intensity projection.
// We include a header for the const version of the slice iterator. For writing
// values to the 2D projection image, we use the linear iterator from the
// previous section.  The linear iterator is chosen because it can be set to
// follow the same path in its underlying 2D image that the slice iterator
// follows over each slice of the 3D image.

#include "otbImage.h"
#include "vnl/vnl_math.h"

#include "itkImageSliceConstIteratorWithIndex.h"
#include "itkImageLinearIteratorWithIndex.h"
#include "otbImageFileReader.h"
#include "otbImageFileWriter.h"

int main(int argc, char* argv[])
  //   Verify the number of parameters on the command line.
  if (argc < 4)
    std::cerr << "Missing parameters. " << std::endl;
    std::cerr << "Usage: " << std::endl;
    std::cerr << argv[0] << " inputImageFile outputImageFile projectionDirection" << std::endl;
    return -1;

  // The pixel type is defined as \code{unsigned short}.  For this application,
  // we need two image types, a 3D image for the input, and a 2D image for the
  // intensity projection.

  using PixelType   = unsigned short;
  using ImageType2D = otb::Image<PixelType, 2>;
  using ImageType3D = otb::Image<PixelType, 3>;

  //  A slice iterator type is defined to walk the input image.

  using LinearIteratorType = itk::ImageLinearIteratorWithIndex<ImageType2D>;
  using SliceIteratorType  = itk::ImageSliceConstIteratorWithIndex<ImageType3D>;

  using ReaderType = otb::ImageFileReader<ImageType3D>;
  using WriterType = otb::ImageFileWriter<ImageType2D>;

  ImageType3D::ConstPointer inputImage;
  ReaderType::Pointer       reader = ReaderType::New();
    inputImage = reader->GetOutput();
  catch (itk::ExceptionObject& err)
    std::cout << "ExceptionObject caught !" << std::endl;
    std::cout << err << std::endl;
    return -1;

  // The projection direction is read from the command line. The projection image
  // will be the size of the 2D plane orthogonal to the projection direction.
  // Its spanning vectors are the two remaining coordinate axes in the volume.
  // These axes are recorded in the \code{direction} array.

  unsigned int projectionDirection = static_cast<unsigned int>(::atoi(argv[3]));

  unsigned int i, j;
  unsigned int direction[2];
  for (i = 0, j = 0; i < 3; ++i)
    if (i != projectionDirection)
      direction[j] = i;

  // The \code{direction} array is now used to define the projection image size
  // based on the input image size.  The output image is created so that its
  // common dimension(s) with the input image are the same size.  For example,
  // if we project along the $x$ axis of the input, the size and origin of the
  // $y$ axes of the input and output will match.  This makes the code slightly
  // more complicated, but prevents a counter-intuitive rotation of the output.

  ImageType2D::RegionType            region;
  ImageType2D::RegionType::SizeType  size;
  ImageType2D::RegionType::IndexType index;

  ImageType3D::RegionType requestedRegion = inputImage->GetRequestedRegion();

  index[direction[0]]     = requestedRegion.GetIndex()[direction[0]];
  index[1 - direction[0]] = requestedRegion.GetIndex()[direction[1]];
  size[direction[0]]      = requestedRegion.GetSize()[direction[0]];
  size[1 - direction[0]]  = requestedRegion.GetSize()[direction[1]];


  ImageType2D::Pointer outputImage = ImageType2D::New();


  // Next we create the necessary iterators.  The const slice iterator walks
  // the 3D input image, and the non-const linear iterator walks the 2D output
  // image. The iterators are initialized to walk the same linear path through
  // a slice.  Remember that the \emph{second} direction of the slice iterator
  // defines the direction that linear iteration walks within a slice.

  SliceIteratorType  inputIt(inputImage, inputImage->GetRequestedRegion());
  LinearIteratorType outputIt(outputImage, outputImage->GetRequestedRegion());


  outputIt.SetDirection(1 - direction[0]);

  // Now we are ready to compute the projection.  The first step is to initialize
  // all of the projection values to their nonpositive minimum value.  The
  // projection values are then updated row by row from the first slice of the
  // input.  At the end of the first slice, the input iterator steps to the first
  // row in the next slice, while the output iterator, whose underlying image
  // consists of only one slice, rewinds to its first row.  The process repeats
  // until the last slice of the input is processed.

  while (!outputIt.IsAtEnd())
    while (!outputIt.IsAtEndOfLine())
      outputIt.Set(itk::NumericTraits<unsigned short>::NonpositiveMin());


  while (!inputIt.IsAtEnd())
    while (!inputIt.IsAtEndOfSlice())
      while (!inputIt.IsAtEndOfLine())
        outputIt.Set(vnl_math_max(outputIt.Get(), inputIt.Get()));

  WriterType::Pointer writer = WriterType::New();
  catch (itk::ExceptionObject& err)
    std::cout << "ExceptionObject caught !" << std::endl;
    std::cout << err << std::endl;
    return -1;

  // Running this example code on the 3D image
  // \code{Examples/Data/BrainProtonDensity3Slices.mha} using the $z$-axis as
  // the axis of projection gives the image shown in
  // Figure~\ref{fig:ImageSliceIteratorWithIndexOutput}.
  // \begin{figure}
  // \centering
  // \includegraphics[width=0.4\textwidth]{ImageSliceIteratorWithIndexOutput.eps}
  // \itkcaption[Maximum intensity projection using ImageSliceIteratorWithIndex]{The
  // maximum intensity projection through three slices of a volume.}
  // \protect\label{fig:ImageSliceIteratorWithIndexOutput}
  // \end{figure}
  // \index{itk::Image\-Slice\-Iterator\-With\-Index!example of using|)}

  return EXIT_SUCCESS;